Heretofore, in a refrigerator, for example, a compression-type refrigeration cycle comprising a compressor, a condenser, an expansion valve and an evaporator, HFCs (hydrofluorocarbons), for example, 1,1,1,2-tetrafluoroethane (R134a) as well as other various types of so-called alternatives for chlorofluorocarbons that are free from environmental pollution have been used as refrigerants. However, such HFCs also involve some problems in that their risk of global warming is high, and recently, use of refrigerants free from such problems has become taken into consideration.
As refrigerants having few influences on global warming, R32 (difluoromethane) refrigerant has also been taken into consideration as an alternative refrigerant, in addition to carbonic acid gas (carbon dioxide), ammonia, hydrocarbon gas.
For example, carbonic acid gas (carbon dioxide) is harmless to the environments, but has problems in that the pressure thereof is high as compared with that of HFC, etc. Use of ammonia and hydrocarbons is not still popular from the viewpoint of toxicity and flammability thereof.
On the other hand, R32 refrigerant is excellent in point of the performance thereof as a refrigerant in that the global warming coefficient of R32 refrigerant is lower than that of HFC refrigerant and therefore, its applicability as a next-generation refrigerant is now under investigation.
A compression-type refrigerator generally comprises at least a compressor, a condenser, an expansion mechanism (such as an expansion valve) and an evaporator, and is constructed in such that a mixed liquid composed of a refrigerator oil that is a lubricant oil for refrigeration apparatuses and a refrigerant is circulated through a closed system including these devices. In such a compression-type refrigerator, in general, a high temperature is established in a compressor while a low temperature is established in a cooler, though depending upon the devices used. Therefore, it is necessary that the refrigerant and the lubricating oil can be circulated through the system without causing a phase separation in a wide temperature range encompassing from the low temperature to the high temperature.
In general, the temperature range in which a refrigerant and a lubricating oil are kept dissolved with no phase separation is preferably from lower than −20° C. to 0° C. or higher. Phase separation, if occurring during refrigerator operation, would have significant negative influences on the life and the efficiency of the apparatus. For example, if phase separation between a refrigerant and a lubricating oil occurs in a compressor section, lubricity in a moving part will become insufficient so that seizing and other problems will occur to considerably reduce the service life of the apparatus. On the other hand, if phase separation occurs in an evaporator, the heat exchange efficiency is reduced because of the presence of a highly viscous lubricating oil. In addition, a lubricating oil for compression-type refrigerators is used for the purpose of lubricating the moving part of a refrigerator, and therefore the lubricating performance thereof is naturally important.
Accordingly, development of a novel refrigerator oil is being advanced suitable for use along with R32 refrigerant, and polyvinyl ether (PVE) is considered as one candidate for it.
However, in case where R32 refrigerant is used in low-temperature devices and when heretofore known PVE is used as a lubricating oil therein, the miscibility of the two is insufficient. Consequently, in case where R32 refrigerant is used in low-temperature devices, the lubricating oil to be used therein is desired to be miscible with the refrigerant with no separation even at a temperature lower than −20° C., and various investigations have heretofore been made.
For example, a lubricating oil for refrigerators using a polyvinyl ether compound having a carbon/oxygen molar ratio (C/O molar ratio) of from 4.2 to 7.0 is disclosed (for example, see PTL 1). The reference describes the miscibility of the compound with R32 refrigerant, but the two-phase separation temperature on the low temperature side of the compound is high, and therefore the miscibility of the two is not always sufficient for use in low-temperature devices.
Use of a polyvinyl ether compound with an oxyalkylene structure introduced into the side chain thereof as a lubricating oil for refrigerators has been investigated (for example, see PTL 2). The reference discloses miscibility of the compound with R32 refrigerant, but the miscibility of the two is not necessarily sufficient.
Further, use of a polyvinyl ether copolymer copolymerized with a structural unit having different ether structures in the side chains thereof, as a lubricating oil for refrigerators has also been investigated (for example, see PTL 3). Also in this case, however, the miscibility between the polyvinyl ether compound and R32 refrigerant is not necessarily sufficient.
On the other hand, a lubricating oil for refrigerators, comprising a compound having a polyalkylene glycol unit and a polyvinyl ether unit in the molecule is disclosed (for example, see PTLs 4 and 5), but this is a lubricating oil for CO2 refrigerant, and no miscibility data thereof with R32 refrigerant are disclosed in the references.
PTL 5 discloses a technique that a polyvinyl ether compound having a C/O molar ratio of 4.0 or less has an excellent miscibility with natural refrigerants (CO2, NH3, C3H8), but refers to nothing relating to R32 refrigerant.
As in the above, the lubricant oils heretofore disclosed all have a problem of separation from R32 refrigerant at up to −20° C.